Slab microtomy combined with Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance... more Slab microtomy combined with Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR) analysis is a useful tool to determine the chemical depth profiles of multilayer coating systems. This approach allows relatively small steps (down to ~5 μm per section) to be used to document chemical composition throughout a coating system or at/near a given locus of interest such as a clearcoat/basecoat interface. In this work, two automotive refinish coating systems were studied: a fast-curing, solvent-borne clearcoat on a standard, non-crosslinked waterborne basecoat, and on a waterborne basecoat containing an isocyanate crosslinker. The objective is to understand the effect of crosslinker addition to the basecoat on the interlayer diffusion of coating materials and the chemical composition of the clearcoat/basecoat system, particularly at the clearcoat/basecoat interface and inside the basecoat. Analysis of depth profiling results shows that in the unmodified basecoat there is a limited amount of isocyanate diffusing from the clearcoat to the basecoat to convert to urea and urethane groups, compared to the system with a crosslinker-containing basecoat. When a crosslinker solution (i.e., crosslinker and solvent) is added to the basecoat, it leads to a more gradual change of the chemical compositions going from the clearcoat to the basecoat. The addition of the crosslinker solution into the basecoat not only helps in crosslinking the basecoat, it also promotes diffusion of the clearcoat materials into the basecoat, making the interface and basecoat stronger than the system using the standard basecoat. The difference of these two coating systems helps to further understand the effect of adding isocyanate crosslinker on coating system properties.
In situ FTIR has been widely used to study polymerization kinetics of different reaction systems.... more In situ FTIR has been widely used to study polymerization kinetics of different reaction systems. However, lack of humidity control during these studies has generally limited this technique to ambient or nitrogen atmosphere conditions. In this work, a relatively small humidity control chamber was newly designed and introduced into the original FTIR sample chamber in transmission mode to control the humidity during in situ kinetic studies of coatings over a broad range from 5 to 80% RH. With the current setup, it only takes 2–3 min to get to the desired humidity level. Factors that affect the humidity control and FTIR data collection are discussed. Two reaction systems were selected to study the effect of moisture on the kinetics. One is an aliphatic polyisocyanate reacting with moisture in the air, and the other is an acrylic polyol reacting with a polyisocyanate. Relative humidity was studied at three levels: 15, 45, and 75%. In both cases, the reaction rate increases under higher humidity. Formation of different forms of urea in the first system and formation of both urethane and urea in the second system were analyzed. In the second reaction system, more urea is formed under higher humidity or under a higher NCO/OH molar ratio. The humidity control within the transmission FTIR chamber has turned out to be a useful tool to monitor the effect of humidity on the curing kinetics of organic coatings.
A kinetic gelation model that simulates free-radical network polymerization on a lattice with a s... more A kinetic gelation model that simulates free-radical network polymerization on a lattice with a stochastic kinetic approach to enable real time calculation was used to assess how initiation rate and primary cyclization affect the overall kinetics of polymerization of difunctional monomers. Changes that cause a more uniform distribution of reacted siteshigher initiation rate or less primary cyclizationincrease the accessibility of free radicals to functional groups, lower the fraction of trapped radicals, and consequently raise the apparent propagation rate constant. On the other hand, the final conversion, determined by kinetic chain length at a given initiator concentration, drops when termination becomes more severe such as under higher initiation rate or when radical trapping worsens such as under enhanced cyclization. In addition, the model simulates the contribution of pendant functional groups to the formation of different structures. The higher the radical concentration brought by higher initiation...
Polymerization kinetics and stress development were measured during the UV curing of multifunctio... more Polymerization kinetics and stress development were measured during the UV curing of multifunctional acrylate and methacrylate coatings by photodifferential scanning calorimetry and a cantilever deflection method. Higher light intensity leads to higher double-bond conversion but unfortunately also to higher stress. Higher monomer functionality unfortunately leads both to lower conversion and to higher stress; substituting methacrylate for acrylate does likewise. Longer monomer chain length and more plasticizer, though, lead both to higher conversion and to lower stress. In all cases, it is shown that significant stress starts to develop only late in reactionsat the vitrification conversion. The vitrification conversion falls as more rigid networks are formed (with higher functionality, shorter monomer chain length, lower plasticizer concentration, or methacrylate rather than acrylate), but it is not affected by the light intensity. After vitrificationsin the vitrified statesthe stress rises monotonically with conversion. The rate of stress growth with conversion in the vitrified state rises with higher monomer functionality, shorter monomer chain length, or lower plasticizer concentration. It also rises when methacrylate is substituted for acrylate. These trends of stress growth in the vitrified state are consistent with an increase in the elastic modulus as more rigid networks are formed.
An improved kinetic model is presented which accounts for radical trapping during the photopolyme... more An improved kinetic model is presented which accounts for radical trapping during the photopolymerization of multifunctional monomers such as diacrylates and dimethacrylates. Following earlier suggestions, the model assumes that trapping of radicals behaves as a unimolecular first-order reaction. The novel feature is that the trapping rate constant is presumed to increase exponentially with the inverse of the free volume; this treatment is qualitatively consistent with the free volume dependence previously proposed for the other rate constants. This improved model predicts the experimental reaction rate trends as well as previous models developed in the literature; more importantly, though, this improved model newly predicts, as no other model has, the following experimental trends in the trapped and active radical concentrations: (1) that the active radical concentration passes through a maximum while the trapped radical concentration increases monotonically; (2) that a higher light intensity leads to a lower fraction of trapped radicals at a given conversion of functional groups but to a higher trapped radical concentration at the end of the reaction. Moreover, unlike its antecedents, the improved model correctly predicts that the polymerization rate depends more on light intensity the higher the conversion and that higher light intensity can lead to a higher final conversion.
A rigidity percolation model was developed to simulate the growth of elastically active bonds and... more A rigidity percolation model was developed to simulate the growth of elastically active bonds and evolution of Young’s modulus during free-radical crosslinking polymerization. The polymerization process was followed by a kinetic gelation modeling. The bonds between monomer sites that form a network were represented as beams with rigid joints. Young’s modulus appears at the percolation threshold, and then monotonically rises with conversion. As the initiation rate is raised, Young’s modulus appears at a higher conversion, but it grows faster beyond the percolation threshold. Consequently, Young’s moduli attained at high conversions are not affected by the initiation rate. As primary cyclization is enhanced, Young’s modulus appears at a higher conversion due to delayed percolation threshold. The difference of Young’s modulus at different levels of primary cyclization is most significant at the percolation threshold regions. After that the difference becomes smaller even though it is still appreciable. The formation of elastically active bonds follows a similar trend as the Young’s modulus to which they give rise. Furthermore, the dependence of Young’s modulus on the number of elastically active bonds indicates that changing the initiation rate does not affect their bonding structure, and that enhancing primary cyclization causes them to form less rigid bonding structure because of its heterogeneity. Finally, irregular distribution of bonds in the network causes stress to be higher in some and lower in others by orders of magnitude.
ABSTRACT To evaluate the long-term performance of transparent conductive oxides (TCOs) in thin-fi... more ABSTRACT To evaluate the long-term performance of transparent conductive oxides (TCOs) in thin-film PV applications, electrochemical corrosion testing was carried out on three fluorine-doped tin oxide films on glass. The test assesses the TCO's susceptibility to delamination in thin-film PV modules. One TCO tested passed the test and the other two failed the test due to delamination and cracking. An X-ray diffraction (XRD) technique was used to determine residual film stress of the TCO films. Compressive stress was found in all the TCO films. The lowest stress was at -723 MPa and the highest stress was at Ȓ1173 MPa. Films with higher residual stresses had more areas of delamination. It is proposed that sodium ions formed at the TCO/glass interface and water diffusion into the film from the electrochemical corrosion test initiate straight-sided blister formation or buckling. When the compressive residual stress in the film is high, buckling-driven delamination occurs. This work suggests that both the electrochemical corrosion test and the XRD film stress analysis may be useful methods to rank the long-term performance of TCOs.
The chapter describes the internal stress development in reactive coatings during the process of ... more The chapter describes the internal stress development in reactive coatings during the process of film formation over a long period of time, e.g., well over 1000 h. Measurements of internal stress were made in situ using a cantilever method. Deflection of the cantilever was measured by two means: the traditional optical microscope method and a novel capacitive sensor method. The capacitive sensor approach was developed to determine the stress of coatings with higher film thickness. It offers unique advantages of high sensitivity and ease of testing multiple coatings simultaneously. The effects of acrylic polyol binder structure on the stress development of polyurethane coatings were examined in detail. The acrylic polyol binders are copolymers with defined structures synthesized by group transfer polymerization. The effects of copolymer structure, such as rigid vs. less rigid segments in the main chain, copolymer molecular weight, and crosslink density of the formed polyurethane network on the internal stress are discussed. Applying rigid segments in the polymeric chain leads to higher stress mainly in the rubbery region and around the vitrification point of the coating. Increasing the crosslinking density increases the plateau value of internal stress. Molecular weight of the copolymer does not affect the internal stress. Addition of a polyester polyol with low glass transition temperature can effectively lower the stress of a polyurethane coating. Addition of solvent to a polyurea coating with 100% solids was also investigated. Finally, the effect of baking, i.e., curing at an elevated temperature, was studied on a polyurea/polyurethane coating. These studies and the capacitive sensor approach developed will help understand long-term performance of reactive coatings.
In situ FTIR has been widely used to study polymerization kinetics of different reaction systems.... more In situ FTIR has been widely used to study polymerization kinetics of different reaction systems. However, lack of humidity control during these studies has generally limited this technique to ambient or nitrogen atmosphere conditions. In this work, a relatively small humidity control chamber was newly designed and introduced into the original FTIR sample chamber in transmission mode to control the humidity during in situ kinetic studies of coatings over a broad range from 5 to 80% RH. With the current setup, it only takes 2–3 min to get to the desired humidity level. Factors that affect the humidity control and FTIR data collection are discussed. Two reaction systems were selected to study the effect of moisture on the kinetics. One is an aliphatic polyisocyanate reacting with moisture in the air, and the other is an acrylic polyol reacting with a polyisocyanate. Relative humidity was studied at three levels: 15, 45, and 75%. In both cases, the reaction rate increases under higher ...
Strengthening of glass is traditionally achieved us ing techniques like thermal tempering and ion... more Strengthening of glass is traditionally achieved us ing techniques like thermal tempering and ion exchange. Both approaches create compressive stres s on the glass surface for strengthening. However, the tendency to create surface distortion in the tempering process, the very slow rates of ion exchange, and the significant energy costs i nvolved in both approaches require new ways of strengthening glass. Applying a coating to glas s surfaces or edges has become more attractive for glass strengthening due to the fact that it is ea ier to implement and has the capability to render other properties to the coated surface.
A rigidity percolation model was developed to simulate the growth of elastically active bonds and... more A rigidity percolation model was developed to simulate the growth of elastically active bonds and evolution of Young’s modulus during free-radical crosslinking polymerization. The polymerization process was followed by a kinetic gelation modeling. The bonds between monomer sites that form a network were represented as beams with rigid joints. Young’s modulus appears at the percolation threshold, and then monotonically rises with conversion. As the initiation rate is raised, Young’s modulus appears at a higher conversion, but it grows faster beyond the percolation threshold. Consequently, Young’s moduli attained at high conversions are not affected by the initiation rate. As primary cyclization is enhanced, Young’s modulus appears at a higher conversion due to delayed percolation threshold. The difference of Young’s modulus at different levels of primary cyclization is most significant at the percolation threshold regions. After that the difference becomes smaller even though it is s...
A rigidity percolation model was developed to simulate the growth of elastically active bonds and... more A rigidity percolation model was developed to simulate the growth of elastically active bonds and evolution of Young’s modulus during free-radical crosslinking polymerization. The polymerization process was followed by a kinetic gelation modeling. The bonds between monomer sites that form a network were represented as beams with rigid joints. Young’s modulus appears at the percolation threshold, and then monotonically rises with conversion. As the initiation rate is raised, Young’s modulus appears at a higher conversion, but it grows faster beyond the percolation threshold. Consequently, Young’s moduli attained at high conversions are not affected by the initiation rate. As primary cyclization is enhanced, Young’s modulus appears at a higher conversion due to delayed percolation threshold. The difference of Young’s modulus at different levels of primary cyclization is most significant at the percolation threshold regions. After that the difference becomes smaller even though it is still appreciable. The formation of elastically active bonds follows a similar trend as the Young’s modulus to which they give rise. Furthermore, the dependence of Young’s modulus on the number of elastically active bonds indicates that changing the initiation rate does not affect their bonding structure, and that enhancing primary cyclization causes them to form less rigid bonding structure because of its heterogeneity. Finally, irregular distribution of bonds in the network causes stress to be higher in some and lower in others by orders of magnitude.
Polymerization kinetics and stress development were measured during the UV curing of multifunctio... more Polymerization kinetics and stress development were measured during the UV curing of multifunctional acrylate and methacrylate coatings by photodifferential scanning calorimetry and a cantilever deflection method. Higher light intensity leads to higher double-bond conversion but unfortunately also to higher stress. Higher monomer functionality unfortunately leads both to lower conversion and to higher stress; substituting methacrylate for acrylate does likewise. Longer monomer chain length and more plasticizer, though, lead both to higher conversion and to lower stress. In all cases, it is shown that significant stress starts to develop only late in reactionsat the vitrification conversion. The vitrification conversion falls as more rigid networks are formed (with higher functionality, shorter monomer chain length, lower plasticizer concentration, or methacrylate rather than acrylate), but it is not affected by the light intensity. After vitrificationsin the vitrified statesthe stress rises monotonically with conversion. The rate of stress growth with conversion in the vitrified state rises with higher monomer functionality, shorter monomer chain length, or lower plasticizer concentration. It also rises when methacrylate is substituted for acrylate. These trends of stress growth in the vitrified state are consistent with an increase in the elastic modulus as more rigid networks are formed.
Edge-strengthening is a novel technology used to strengthen glass by applying a coating only on t... more Edge-strengthening is a novel technology used to strengthen glass by applying a coating only on the edges. In this work, edge flaws of flat glass articles were examined in detail via scanning electron microscope (SEM) imaging. Then the effect of using weatherable acrylate coatings on edge-strengthening of flat glass was determined. Four-point bending measurement showed that the coatings provided more than twofold increase of the mean flexural strength and a factor of about two increase of the tensile stress needed to reach 0.8% cumulative probability of failure. The coatings were found to cover the flaw zone at the glass edges and partially fill in the cracks. Different surface treatments led to different levels of strengthening, indicating the importance of coating adhesion. The coating's thermal and mechanical properties affected the extent of strengthening effect. A coating formulation with a higher glass transition temperature tended to provide a better strengthening effect, indicating the importance of closure stress within cracks generated during film curing process. Challenges of applying the edge-strengthening technology are also discussed.
Slab microtomy combined with Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance... more Slab microtomy combined with Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR) analysis is a useful tool to determine the chemical depth profiles of multilayer coating systems. This approach allows relatively small steps (down to ~5 μm per section) to be used to document chemical composition throughout a coating system or at/near a given locus of interest such as a clearcoat/basecoat interface. In this work, two automotive refinish coating systems were studied: a fast-curing, solvent-borne clearcoat on a standard, non-crosslinked waterborne basecoat, and on a waterborne basecoat containing an isocyanate crosslinker. The objective is to understand the effect of crosslinker addition to the basecoat on the interlayer diffusion of coating materials and the chemical composition of the clearcoat/basecoat system, particularly at the clearcoat/basecoat interface and inside the basecoat. Analysis of depth profiling results shows that in the unmodified basecoat there is a limited amount of isocyanate diffusing from the clearcoat to the basecoat to convert to urea and urethane groups, compared to the system with a crosslinker-containing basecoat. When a crosslinker solution (i.e., crosslinker and solvent) is added to the basecoat, it leads to a more gradual change of the chemical compositions going from the clearcoat to the basecoat. The addition of the crosslinker solution into the basecoat not only helps in crosslinking the basecoat, it also promotes diffusion of the clearcoat materials into the basecoat, making the interface and basecoat stronger than the system using the standard basecoat. The difference of these two coating systems helps to further understand the effect of adding isocyanate crosslinker on coating system properties.
In situ FTIR has been widely used to study polymerization kinetics of different reaction systems.... more In situ FTIR has been widely used to study polymerization kinetics of different reaction systems. However, lack of humidity control during these studies has generally limited this technique to ambient or nitrogen atmosphere conditions. In this work, a relatively small humidity control chamber was newly designed and introduced into the original FTIR sample chamber in transmission mode to control the humidity during in situ kinetic studies of coatings over a broad range from 5 to 80% RH. With the current setup, it only takes 2–3 min to get to the desired humidity level. Factors that affect the humidity control and FTIR data collection are discussed. Two reaction systems were selected to study the effect of moisture on the kinetics. One is an aliphatic polyisocyanate reacting with moisture in the air, and the other is an acrylic polyol reacting with a polyisocyanate. Relative humidity was studied at three levels: 15, 45, and 75%. In both cases, the reaction rate increases under higher humidity. Formation of different forms of urea in the first system and formation of both urethane and urea in the second system were analyzed. In the second reaction system, more urea is formed under higher humidity or under a higher NCO/OH molar ratio. The humidity control within the transmission FTIR chamber has turned out to be a useful tool to monitor the effect of humidity on the curing kinetics of organic coatings.
A kinetic gelation model that simulates free-radical network polymerization on a lattice with a s... more A kinetic gelation model that simulates free-radical network polymerization on a lattice with a stochastic kinetic approach to enable real time calculation was used to assess how initiation rate and primary cyclization affect the overall kinetics of polymerization of difunctional monomers. Changes that cause a more uniform distribution of reacted siteshigher initiation rate or less primary cyclizationincrease the accessibility of free radicals to functional groups, lower the fraction of trapped radicals, and consequently raise the apparent propagation rate constant. On the other hand, the final conversion, determined by kinetic chain length at a given initiator concentration, drops when termination becomes more severe such as under higher initiation rate or when radical trapping worsens such as under enhanced cyclization. In addition, the model simulates the contribution of pendant functional groups to the formation of different structures. The higher the radical concentration brought by higher initiation...
Polymerization kinetics and stress development were measured during the UV curing of multifunctio... more Polymerization kinetics and stress development were measured during the UV curing of multifunctional acrylate and methacrylate coatings by photodifferential scanning calorimetry and a cantilever deflection method. Higher light intensity leads to higher double-bond conversion but unfortunately also to higher stress. Higher monomer functionality unfortunately leads both to lower conversion and to higher stress; substituting methacrylate for acrylate does likewise. Longer monomer chain length and more plasticizer, though, lead both to higher conversion and to lower stress. In all cases, it is shown that significant stress starts to develop only late in reactionsat the vitrification conversion. The vitrification conversion falls as more rigid networks are formed (with higher functionality, shorter monomer chain length, lower plasticizer concentration, or methacrylate rather than acrylate), but it is not affected by the light intensity. After vitrificationsin the vitrified statesthe stress rises monotonically with conversion. The rate of stress growth with conversion in the vitrified state rises with higher monomer functionality, shorter monomer chain length, or lower plasticizer concentration. It also rises when methacrylate is substituted for acrylate. These trends of stress growth in the vitrified state are consistent with an increase in the elastic modulus as more rigid networks are formed.
An improved kinetic model is presented which accounts for radical trapping during the photopolyme... more An improved kinetic model is presented which accounts for radical trapping during the photopolymerization of multifunctional monomers such as diacrylates and dimethacrylates. Following earlier suggestions, the model assumes that trapping of radicals behaves as a unimolecular first-order reaction. The novel feature is that the trapping rate constant is presumed to increase exponentially with the inverse of the free volume; this treatment is qualitatively consistent with the free volume dependence previously proposed for the other rate constants. This improved model predicts the experimental reaction rate trends as well as previous models developed in the literature; more importantly, though, this improved model newly predicts, as no other model has, the following experimental trends in the trapped and active radical concentrations: (1) that the active radical concentration passes through a maximum while the trapped radical concentration increases monotonically; (2) that a higher light intensity leads to a lower fraction of trapped radicals at a given conversion of functional groups but to a higher trapped radical concentration at the end of the reaction. Moreover, unlike its antecedents, the improved model correctly predicts that the polymerization rate depends more on light intensity the higher the conversion and that higher light intensity can lead to a higher final conversion.
A rigidity percolation model was developed to simulate the growth of elastically active bonds and... more A rigidity percolation model was developed to simulate the growth of elastically active bonds and evolution of Young’s modulus during free-radical crosslinking polymerization. The polymerization process was followed by a kinetic gelation modeling. The bonds between monomer sites that form a network were represented as beams with rigid joints. Young’s modulus appears at the percolation threshold, and then monotonically rises with conversion. As the initiation rate is raised, Young’s modulus appears at a higher conversion, but it grows faster beyond the percolation threshold. Consequently, Young’s moduli attained at high conversions are not affected by the initiation rate. As primary cyclization is enhanced, Young’s modulus appears at a higher conversion due to delayed percolation threshold. The difference of Young’s modulus at different levels of primary cyclization is most significant at the percolation threshold regions. After that the difference becomes smaller even though it is still appreciable. The formation of elastically active bonds follows a similar trend as the Young’s modulus to which they give rise. Furthermore, the dependence of Young’s modulus on the number of elastically active bonds indicates that changing the initiation rate does not affect their bonding structure, and that enhancing primary cyclization causes them to form less rigid bonding structure because of its heterogeneity. Finally, irregular distribution of bonds in the network causes stress to be higher in some and lower in others by orders of magnitude.
ABSTRACT To evaluate the long-term performance of transparent conductive oxides (TCOs) in thin-fi... more ABSTRACT To evaluate the long-term performance of transparent conductive oxides (TCOs) in thin-film PV applications, electrochemical corrosion testing was carried out on three fluorine-doped tin oxide films on glass. The test assesses the TCO's susceptibility to delamination in thin-film PV modules. One TCO tested passed the test and the other two failed the test due to delamination and cracking. An X-ray diffraction (XRD) technique was used to determine residual film stress of the TCO films. Compressive stress was found in all the TCO films. The lowest stress was at -723 MPa and the highest stress was at Ȓ1173 MPa. Films with higher residual stresses had more areas of delamination. It is proposed that sodium ions formed at the TCO/glass interface and water diffusion into the film from the electrochemical corrosion test initiate straight-sided blister formation or buckling. When the compressive residual stress in the film is high, buckling-driven delamination occurs. This work suggests that both the electrochemical corrosion test and the XRD film stress analysis may be useful methods to rank the long-term performance of TCOs.
The chapter describes the internal stress development in reactive coatings during the process of ... more The chapter describes the internal stress development in reactive coatings during the process of film formation over a long period of time, e.g., well over 1000 h. Measurements of internal stress were made in situ using a cantilever method. Deflection of the cantilever was measured by two means: the traditional optical microscope method and a novel capacitive sensor method. The capacitive sensor approach was developed to determine the stress of coatings with higher film thickness. It offers unique advantages of high sensitivity and ease of testing multiple coatings simultaneously. The effects of acrylic polyol binder structure on the stress development of polyurethane coatings were examined in detail. The acrylic polyol binders are copolymers with defined structures synthesized by group transfer polymerization. The effects of copolymer structure, such as rigid vs. less rigid segments in the main chain, copolymer molecular weight, and crosslink density of the formed polyurethane network on the internal stress are discussed. Applying rigid segments in the polymeric chain leads to higher stress mainly in the rubbery region and around the vitrification point of the coating. Increasing the crosslinking density increases the plateau value of internal stress. Molecular weight of the copolymer does not affect the internal stress. Addition of a polyester polyol with low glass transition temperature can effectively lower the stress of a polyurethane coating. Addition of solvent to a polyurea coating with 100% solids was also investigated. Finally, the effect of baking, i.e., curing at an elevated temperature, was studied on a polyurea/polyurethane coating. These studies and the capacitive sensor approach developed will help understand long-term performance of reactive coatings.
In situ FTIR has been widely used to study polymerization kinetics of different reaction systems.... more In situ FTIR has been widely used to study polymerization kinetics of different reaction systems. However, lack of humidity control during these studies has generally limited this technique to ambient or nitrogen atmosphere conditions. In this work, a relatively small humidity control chamber was newly designed and introduced into the original FTIR sample chamber in transmission mode to control the humidity during in situ kinetic studies of coatings over a broad range from 5 to 80% RH. With the current setup, it only takes 2–3 min to get to the desired humidity level. Factors that affect the humidity control and FTIR data collection are discussed. Two reaction systems were selected to study the effect of moisture on the kinetics. One is an aliphatic polyisocyanate reacting with moisture in the air, and the other is an acrylic polyol reacting with a polyisocyanate. Relative humidity was studied at three levels: 15, 45, and 75%. In both cases, the reaction rate increases under higher ...
Strengthening of glass is traditionally achieved us ing techniques like thermal tempering and ion... more Strengthening of glass is traditionally achieved us ing techniques like thermal tempering and ion exchange. Both approaches create compressive stres s on the glass surface for strengthening. However, the tendency to create surface distortion in the tempering process, the very slow rates of ion exchange, and the significant energy costs i nvolved in both approaches require new ways of strengthening glass. Applying a coating to glas s surfaces or edges has become more attractive for glass strengthening due to the fact that it is ea ier to implement and has the capability to render other properties to the coated surface.
A rigidity percolation model was developed to simulate the growth of elastically active bonds and... more A rigidity percolation model was developed to simulate the growth of elastically active bonds and evolution of Young’s modulus during free-radical crosslinking polymerization. The polymerization process was followed by a kinetic gelation modeling. The bonds between monomer sites that form a network were represented as beams with rigid joints. Young’s modulus appears at the percolation threshold, and then monotonically rises with conversion. As the initiation rate is raised, Young’s modulus appears at a higher conversion, but it grows faster beyond the percolation threshold. Consequently, Young’s moduli attained at high conversions are not affected by the initiation rate. As primary cyclization is enhanced, Young’s modulus appears at a higher conversion due to delayed percolation threshold. The difference of Young’s modulus at different levels of primary cyclization is most significant at the percolation threshold regions. After that the difference becomes smaller even though it is s...
A rigidity percolation model was developed to simulate the growth of elastically active bonds and... more A rigidity percolation model was developed to simulate the growth of elastically active bonds and evolution of Young’s modulus during free-radical crosslinking polymerization. The polymerization process was followed by a kinetic gelation modeling. The bonds between monomer sites that form a network were represented as beams with rigid joints. Young’s modulus appears at the percolation threshold, and then monotonically rises with conversion. As the initiation rate is raised, Young’s modulus appears at a higher conversion, but it grows faster beyond the percolation threshold. Consequently, Young’s moduli attained at high conversions are not affected by the initiation rate. As primary cyclization is enhanced, Young’s modulus appears at a higher conversion due to delayed percolation threshold. The difference of Young’s modulus at different levels of primary cyclization is most significant at the percolation threshold regions. After that the difference becomes smaller even though it is still appreciable. The formation of elastically active bonds follows a similar trend as the Young’s modulus to which they give rise. Furthermore, the dependence of Young’s modulus on the number of elastically active bonds indicates that changing the initiation rate does not affect their bonding structure, and that enhancing primary cyclization causes them to form less rigid bonding structure because of its heterogeneity. Finally, irregular distribution of bonds in the network causes stress to be higher in some and lower in others by orders of magnitude.
Polymerization kinetics and stress development were measured during the UV curing of multifunctio... more Polymerization kinetics and stress development were measured during the UV curing of multifunctional acrylate and methacrylate coatings by photodifferential scanning calorimetry and a cantilever deflection method. Higher light intensity leads to higher double-bond conversion but unfortunately also to higher stress. Higher monomer functionality unfortunately leads both to lower conversion and to higher stress; substituting methacrylate for acrylate does likewise. Longer monomer chain length and more plasticizer, though, lead both to higher conversion and to lower stress. In all cases, it is shown that significant stress starts to develop only late in reactionsat the vitrification conversion. The vitrification conversion falls as more rigid networks are formed (with higher functionality, shorter monomer chain length, lower plasticizer concentration, or methacrylate rather than acrylate), but it is not affected by the light intensity. After vitrificationsin the vitrified statesthe stress rises monotonically with conversion. The rate of stress growth with conversion in the vitrified state rises with higher monomer functionality, shorter monomer chain length, or lower plasticizer concentration. It also rises when methacrylate is substituted for acrylate. These trends of stress growth in the vitrified state are consistent with an increase in the elastic modulus as more rigid networks are formed.
Edge-strengthening is a novel technology used to strengthen glass by applying a coating only on t... more Edge-strengthening is a novel technology used to strengthen glass by applying a coating only on the edges. In this work, edge flaws of flat glass articles were examined in detail via scanning electron microscope (SEM) imaging. Then the effect of using weatherable acrylate coatings on edge-strengthening of flat glass was determined. Four-point bending measurement showed that the coatings provided more than twofold increase of the mean flexural strength and a factor of about two increase of the tensile stress needed to reach 0.8% cumulative probability of failure. The coatings were found to cover the flaw zone at the glass edges and partially fill in the cracks. Different surface treatments led to different levels of strengthening, indicating the importance of coating adhesion. The coating's thermal and mechanical properties affected the extent of strengthening effect. A coating formulation with a higher glass transition temperature tended to provide a better strengthening effect, indicating the importance of closure stress within cracks generated during film curing process. Challenges of applying the edge-strengthening technology are also discussed.
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